The present invention generally relates to semiconductor devices and more particularly to a high speed compound semiconductor device having a super lattice buffer.
In compound semiconductor devices, an active layer or a channel layer is formed by using a compound semiconductor, in which electron mobility is high, to attain high speed operation. Therefore, the compound semiconductor devices are important to high speed radio communication network systems employing GHz bands, including cellular phone service systems, and especially high power compound semiconductor devices are desired for base stations of such high speed radio communication networks.
FIG. 1 shows the structure of a conventional MESFET 10 used as an output transistor at the final stage in a base station of a high-speed radio communication network system.
Referring to FIG. 1, the MESFET 10 is formed on a semi-insulating GaAs substrate 11, and includes an undoped GaAs buffer layer 12 formed epitaxially on the GaAs substrate 11, and an n-type GaAs channel layer 13 formed epitaxially on the buffer layer 12. On the channel layer 13 is formed a gate electrode 14G which corresponds to a channel region. A source electrode 14S and a drain electrode 14D are formed on opposite sides of the gate electrode 14G respectively, as shown in FIG. 1.
FIG. 2 is a chart illustrating the characteristic curves of drain-source currents Ids versus drain-source voltages Vds of the MESFET 10 of FIG. 1. In this chart, a gate-source voltage Vgs is varied incrementally by 200 mV each incremented to give many curves. The vertical axis represents the Ids of the MESFET 10 with each division on the scale equaling 500 mA, and the horizontal axis represents the drain-source voltage Vds with each division of the scale equaling 2V.
Referring to FIG. 2, the drain-source current Ids increases as the gate-source voltage Vgs increases. But after the Ids reaches the saturation region, it suddenly drops as the drain-source voltage Vds increases. It is known that this sudden drop of the saturated drain-source current Ids is due to the Gunn Effect in the MESFET. As a result, the MESFET 10 has a limitation on high frequency power that can be obtained. The chart of FIG. 2 shows the characteristic curves of the conventional MESFET 10 employing the semi-insulating GaAs substrate 11, and the substrate 11 has a high resistivity of more than 1×108 Ohm-cm.
When high resistance semi-insulating GaAs is used as the substrate 11 shown in FIG. 1 and a high voltage is applied to the channel layer 13, it will create multiple electric double regions comprising electron accumulation regions and electron depletion regions abutting on each other in turn. This is a domain structure and Gunn oscillation occurs.
FIG. 3 is a schematic chart illustrating the characteristic curves shown in FIG. 2 together with the load line of the MESFET 10.
Referring to FIG. 3, the actual operational region of the MESFET 10 is located in the area where the saturated drain-source current Ids suddenly drops. Accordingly, the MESFFET 10 cannot provide a desired high power output. In FIG. 3, the minimum current of the saturated drain-source current Ids in the Gunn oscillation area is represented by Idosc. The minimum saturated drain-source current Idosc is about 2400 mA in the FIG. 2 example.
Accordingly, conventional compound semiconductor devices cannot provide enough power when they are used for high power applications such as a base station output stage in high-speed radio communication systems. Various improvements in device structures have been tried in order to solve the above problems.